1. Field of the Invention
The present invention relates to a method of submerged arc-welding of a thick steel plate having a thickness exceeding 30 mm with a large heat input.
2. Description of the Background Art
Recently, steel plates having a thickness exceeding 30 mm have been used as the building structural members, especially as structural members for high rise buildings.
When the thick steel plates having thicknesses exceeding 30 mm are welded, joints having good appearance, and no internal defects should be produced without difficulty. In order to meet this goal, a method has been usually employed using underlay CO.sub.2 -welding. However, the underlaying CO.sub.2 -welding needs a lot of time and labors, this method is not suitable for welding a large quantity of thick building structural members.
In order to conduct welding with high efficiency, it is preferable that joints be formed by one pass/single layer welding with a large heat input, for example, 100 KJ/cm. If the above cannot be conducted because the thickness of the plate exceeds a certain limit, it is preferable that a multiplicity of passes/single layer welding or multi-layer welding be conducted.
It has been disclosed in Japanese Patent Laid-Open No. 53-108839 that the quantity of weld deposition is increased by using a flux to which iron powder has been added and using a multiplicity of electrodes with a large heat input for the purpose of one-pass/single layer welding a thick steel plate having a thickness exceeding 35 mm. However, the thickness which can be welded by one-pass has a certain limitation even if the above method is employed Furthermore, as the welding heat input increases, the characteristics of the welded joint are adversely affected
On the other hand, when multi-layer submerged arc welding is conducted with a large heat input, the amount of slag increases in proportion to the increase of the heat input, and what is even worse, the slag is difficult to remove. As a result, the removal of slag takes a long time or any remaining slag causes welding defects at the multilayer welding.
In order to overcome the above-described problems, a method has been disclosed in Japanese Patent Laid-Open No. 63-1925593 in which the slag can be readily removed by adding iron powder and adjusting the composition of flux.
However, the multi-layer welding of thick steel plates cannot be conducted efficiently with the above-described method because the thickness of the slag inevitably increases proportionally with the increase in the heat input and the proper heat input region is limited by a groove.
That is, in order to easily remove the slag by the above-described method, the groove angle must be 40. or more. However, it leads to a fact that the cross sectional area of the groove is enlarged excessively in accordance with increase in the plate thickness in the case of the single side welding. As a result, problems arise in that the strength and the toughness of the heat affected zone deteriorates because the heat input is enlarged. On the contrary, if the groove angle is reduced, solidified portions generate cracks because the width is narrowed with respect to the depth of fusion (the height of the penetrated metal) when using the first layer of the multi-layer welding with a large heat input. Because of the problems described above, the multi-layer welding with a large heat input has not been generally used.